The present invention relates to magneto-optical recording media, such as magneto-optical disks, magneto-optical tapes and magneto-optical cards, for use in magneto-optical recording and reproducing devices.
Conventionally, a magneto-optical disk using a magneto-optical recording medium has been practically used as a rewritable optical recording medium. On such a magneto-optical disk, recording and erasure are performed by converging a light beam emitted by a semiconductor laser on the optical recording medium to raise a local temperature of the magneto-optical recording medium. The recorded information is reproduced by converging a light beam with such an intensity that recording and erasure are not effected on the magneto-optical recording medium and identifying the polarization state of the reflected light.
However, such a magneto-optical recording medium has a problem that the reproducing characteristics deteriorate as the recorded bit diameter of a recorded magnetic domain and the distance between the recorded bits decrease with respect to the beam spot diameter of the light beam. This problem is caused by the entry of adjacent recorded bit into the beam spot of the light beam converged on a target recorded bit, which prevents separate reproduction of individual recorded bits.
In order to solve the above problem, there has been a proposed magneto-optical recording medium including a reproducing magnetic layer which shows an in-plane magnetization state at room temperature and a perpendicular magnetization state at temperatures of not lower than a critical temperature, a non-magnetic intermediate layer, and a recording magnetic layer formed of a perpendicular magnetization film for recording information (Japanese laid-open patent application (Tokukaihei) No. 9-180276 (published date: Jul. 11, 1997)(USPN 5,777,953)). Moreover, in order to improve the reproducing characteristics of the magneto-optical recording medium, there has been a proposed magneto-optical recording medium including a reproducing magnetic layer which shows an in-plane magnetization state at room temperature and a perpendicular magnetization state at temperatures of not lower than a critical temperature, an in-plane magnetization layer having its Curie temperature in the vicinity of the critical temperature, a non-magnetic intermediate layer, and a recording magnetic layer formed of a perpendicular magnetization film for recording information (Japanese laid-open patent application (Tokukaihei) No. 9-320134 (published date: Dec. 12, 1997) (U.S. Pat. No. 5,939,187)).
In these magneto-optical recording media, since the reproducing magnetic layer shows an in-plane magnetization state within a temperature range below the critical temperature, the recorded magnetic domain information recorded on the recording magnetic layer is not copied to the reproducing magnetic layer, and thus the recorded magnetic domain information is not reproduced. In contrast, within a temperature range of not lower than the critical temperature, the reproducing magnetic layer shows a perpendicular magnetization state, the recorded magnetic domain information recorded in the recording magnetic layer is copied to the reproducing magnetic layer, and the recorded magnetic domain information is reproduced. Therefore, even when adjacent recorded bit enters into the beam spot of the light beam converged on the reproducing magnetic layer, if the reproducing power of the light beam and the critical temperature at which the reproducing magnetic layer changes into a perpendicular magnetization state are set suitably, the individual recorded bits can be reproduced separately, thereby allowing reproduction of information recorded at high density.
In recent years, however, there has been demand for optical disks with a larger recording capacity. Accordingly, it is required to form a smaller recorded magnetic domain in the recording magnetic layer, copy the recorded magnetic domain to the reproducing magnetic layer, and reproduce the recorded magnetic domain in a stable manner. In the magneto-optical disks disclosed in the above-mentioned publications Nos. 9-180276 and 9-320134, GdFeCo that is used as the reproducing magnetic layer has small perpendicular magnetic anisotropy, and the total magnetization of the reproducing magnetic layer becomes very small and the reproducing power margin becomes narrower with a rise in temperature. Thus, there has been a problem that it is impossible to copy a smaller recorded magnetic domain to the reproducing magnetic layer and reproduce the smaller recorded magnetic domain in a stable manner.
An object of the present invention is to provide a magneto-optical recording medium with a wide reproducing power margin.
In order to achieve the above object, a magneto-optical recording medium of the present invention includes at least: a reproducing magnetic layer composed at least of Gd and Co showing an in-plane magnetization state at room temperature and a transition to a perpendicular magnetization state at temperatures of not lower than a critical temperature; and a recording magnetic layer formed of a perpendicular magnetization film, the reproducing magnetic layer and the recording magnetic layer being magneto-statically coupled at least in the vicinity of the critical temperature, and is characterized in that the reproducing magnetic layer contains at least either Tb or Dy so as to increase the total magnetization at a temperature at which a perpendicular magnetic anisotropy constant and a diamagnetic field energy are equal to each other.
According to this structure, since either a rare-earth metal Tb or Dy capable of increasing the perpendicular magnetic anisotropy constant Ku is contained as the reproducing magnetic layer, it is possible to obtain a greater perpendicular magnetic anisotropy constant Ku compared with GdFeCo. Therefore, even when the total magnetization Ms within a temperature range within which the reproducing magnetic layer shows a perpendicular magnetization state is relatively large, it is possible to bring the reproducing magnetic layer into the perpendicular magnetization state.
Thus, during the copying and reproduction of the magnetization information of the recording magnetic layer onto the reproducing magnetic layer by magnetostatic coupling, the total magnetization Ms of the reproducing magnetic layer in a temperature region where the reproducing magnetic layer shows the perpendicular magnetization state with an increase of temperature is sufficiently large, and magnetostatic coupling forces between the reproducing magnetic layer and the recording magnetic layer are sufficiently strengthen. As a result, the magnetization information of the recording magnetic layer can be copied to the reproducing magnetic layer in a stable manner. Moreover, even when the temperature of the reproducing magnetic layer is further raised by an increase of the reproducing power, since the decrease of the total magnetization of the reproducing magnetic layer is relatively small, magnetostatic coupling forces necessary for copying and reproduction can be obtained, thereby providing a wide reproducing power margin.